The invention relates to Wireless data transmission using High-Speed Downlink Packet Access, denoted HSDPA, in a downlink direction, and especially timer handling in a transmitter such as NodeB. The invention relates also to wireless data transmission using High-Speed Uplink Packet Access, denoted HSUPA, in an uplink direction, and especially timer handling in a transmitter such as the User Equipment (UE). The focus of the invention is upon the use of a timer T1_NodeB for HSDPA traffic in NodeB being similar to a timer T1_UE in the UE for HSUPA traffic.
FIG. 1 schematically shows High-Speed Downlink Shared Channel (HS-DSCH) in NodeB and FIG. 2 schematically shows HS-DSCH in an UE. FIGS. 1 and 2 relate to the downlink direction, i.e. from NodeB to the UE.
With HSDPA, a new acknowledged protocol between the NodeB and the UE is introduced. NodeB buffers incoming downlink end user data and utilizes an internal scheduling entity to determine when to transmit buffered data. The aim of the scheduling decision is that the NodeB continuously receives channel quality estimates from the UE entities. NodeB also has knowledge about UE receiver capabilities.
At a pace of up to 500 times per second NodeB transmits Medium Access Control High Speed, hereinafter called MAC-hs, Protocol Data Units, PDUs, to the UEs. At each 2 ms transmit opportunity NodeB can vary the MAC-hs PDU size depending on buffered amount of data, channel quality estimates, UE capabilities and granted amount of downlink codes available. For each MAC-hs entity, data for one UE or up to fifteen UEs can be scheduled at each 2 ms transmit opportunity utilizing code division among the scheduled UEs. The UE decodes a HSDPA control channel, denoted High-Speed Shared Control Channel (HS-SCCH), and upon a successful CRC checksum the UE continues to decode the HSDPA packet data channel, denoted High-Speed Physical Downlink Shared Channel (HS-PDSCH).
Depending on the outcome of the HS-SCCH and HS-PDSCH the UE transmits a reception feedback back to the peer NodeB. The reception feedback is interpreted by the NodeB sender, and upon a negative feedback indicating a reception failure for the UE NodeB retransmits the data, or in absence of UE feedback, i.e. Discontinuous Transmission DTX, NodeB retransmits the data.
MAC-hs PDUs are numbered by modulo Transport Sequence Number, hereinafter called TSN, cycling through the field 0 to 63. The MAC-hs constitutes of multiple Hybrid-Automatic Repeat-reQuest, hereinafter called HARQ, processes, where each HARQ process transmits a MAC-hs PDU and awaits either an ACKnowledgement, hereinafter called ACK, or Negative Acknowledgement, hereinafter called NACK, meaning that the receiver did not correctly decode the MAC-hs PDU, or absence of feedback, DTX. The round trip time comprises the time from MAC-hs PDU transmission until reception of the feedback ACK/NACK is fixed, except for the air interface propagation delay. Upon the reception of a NACK the MAC-hs sender retransmits the MAC-hs PDU. The reason for having multiple HARQ processes is because the round trip time is in relative terms long so if only one HARQ process is available, the total transmission time t over total connection time T, hereinafter called t/T would be low. This means that; while one process awaits a response, another HARQ process, or multiple HARQ processes, transmits. As a result of this, t/T can be 100 percent.
The MAC-hs protocol is semi-reliable meaning that the MAC-hs entity on the transmitting side can discard MAC-hs PDUs that have been transmitted and possibly retransmit ones or several times to the MAC-hs entity on the receiving side.
The reason for discarding a MAC-hs PDU is to prevent unnecessary retransmissions over the radio link where the MAC-hs receiver has moved to another cell, has powered down, or due to any other reason is not capable of receiving data, and is done either at a predetermined time after first transmission or a maximum number of retransmissions, or at a combination of both.
The MAC-hs receiver at the UE utilizes a receiver window with the purpose of mitigating the effect of when PDUs are received in non-ascending sequence order (which can occur due to retransmissions). Whenever a MAC-hs PDU is successfully decoded with TSN equal, the next_expected_tsn, the receiver can deliver PDUs to the RLC layer. Depending on whether the subsequent TSN number (i.e. next_expected_tsn+1) is already successfully decoded that MAC-hs PDU can also be delivered and so forth. The receiver window is updated accordingly. Delivery to the RLC layer from the MAC-hs protocol is done in sequence.
To recover from the situation where e.g. the sender has discarded a MAC-hs PDU, the receiver utilizes the two below described mechanisms to solve the problem:
1) Timer Based Stall Avoidance:
At the reception of a PDU with TSN>next_expected_tsn the receiver starts a timer denoted T1. When the timer expires the receiver take proper actions to allow for subsequent PDUs to be received. The exact details are described in 3GPP 25.321 chapters 11.6.2.3.2.
This behavior is also illustrated in FIG. 5.
2) Window Based Stall Avoidance:
The receiver window denoted window_size is <32 and comprises all TSNs within the range [highest_received_tsn−window_size+1, highest_received_tsn].